If you trailer/camper does not come with a dedicated Black Tank Flush the you will need to follow these intructions when you want to flush your black tank. The best way to utilize what you have on site is to first Drain the black water tank, then drain the gray water tank so the water from the gray tank flushes the outlet. Close the valve and add 2-3 gallons of water to the black tank and drain the black tank again. You can add detergent/solution to the water for maximum smell removal. Repeat these steps as much as desired.